The present invention relates generally to a method and apparatus for continuous time-variant digital filtering. More particularly, a novel method and apparatus are provided for continuous time-variant digital filtering of seismic data to obtain seismic sections in which complex reverberations can be suppressed.
Geophysical exploration for hydrocarbons and other minerals using the seismic technique comprises imparting seismic energy into the earth and recording the resulting seismic waves, which propagate through the earth after reflections and/or refractions, as electrical signals. The electrical signals are then digitized for subsequent computer implemented processing in order to produce seismograms, i.e., maps of the subsurface geologic structure.
As is well known, any electrical signal is comprised of a number of fundamental component signals of differing amplitude and frequency. Digital filtering is a process for separating the various component signals within an electrical signal. In particular, the electrical signals recorded by geophones in seismic exploration can be considered to represent a time series comprising a reflectivity function and a source wavelet. By filtering the recorded signal, geophysicists can obtain a measure of the earth's reflectivity function which can provide information regarding the subsurface geological structure.
The Wiener filter is one of the most effective tools for the digital reduction of seismic data. It constitutes the keystone of many current deconvolution methods. In one application, this filter can be used to deconvolve a reverberating pulse train into an approximation of a zero delay unit impulse. More generally, it is possible to derive Wiener filters which remove repetitive events having specified periodicities. In such context, the Wiener filter can be viewed as a predictor of coherent energy. Consequently, the phrase "predictive deconvolution" is understood by those in the art as a method of decomposing an electrical signal in which coherent energy is reinforced.
The technique set forth by Burg in U.S. Pat. No. 3,512,127 describes a deconvolution filter to reduce reverberation effects caused by multiple reflections of the imparted seismic energy from near surface reflectors. Additionally, Peacock in U.S. Pat. No. 4,535,417 recognizes that the amplitude and frequency content of the imparted seismic wavelet can decay as a function of time and consequently proposes a time-variant filter using a Hilbert transform and a time-varying instantaneous frequency control function to process electrical signals having a time-varying electrical character. Similarly, others such as Ristow and Kosbahn, "Time-Varying Prediction Filtering by Means of Updating," Geophysical Prospecting, 27, p. 40-61 (1979), and Pann and Shin, "A Class of Convolutional Time-Varying Filters," Geophysics, v. 41, No. 1, February, 1976, p. 28-43, other approaches to developing time-variant filters. However, Applicant is presently aware of no technique for generating a continuous time-variant filter operator which does not require excessive computer memory nor one which does not comprise splicing together discrete time windows of invariantly filtered time windows. Applicant's technique is not constrained to filter type or signal nature. Moreover, applicant's method and apparatus provide for both time-variant prediction distance as well as time-variant filter length.